Of the xylene isomers, para-xylene is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Today, para-xylene is commercially produced by hydrotreating of naphtha (catalytic reforming), steam cracking of naphtha or gas oil, and toluene disproportionation.
One problem with most existing processes for producing xylenes is that they produce a thermodynamic equilibrium mixture of ortho (o)-, meta (m)- and para (p)-xylenes, in which the para-xylene concentration is typically only about 24 wt %. Thus, separation of para-xylene from such mixtures typically requires superfractionation and multistage refrigeration steps. Such processes involve high operational costs and result in only limited yields. There is therefore a continuing need to provide processes for producing xylenes which are highly selective for para-isomers.
It is well-known to manufacture xylenes by the alkylation of toluene and/or benzene with methanol, and, in particular, to selectively make para-xylene (PX) product using zeolite catalyst. See, for instance, U.S. Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and 6,642,426. The terms “para-xylene selectivity”, “para-selective”, and the like, means that para-xylene is produced in amounts greater than is present in a mixture of xylene isomers at thermodynamic equilibrium, which at ordinary processing temperatures is about 24 mol %. Para-xylene selectivity is highly sought after because of the economic importance of para-xylene relative to meta- and ortho-xylene. Although each of the xylene isomers have important and well-known end uses, para-xylene is currently the most economically valuable.
In the process, typically toluene and/or benzene are alkylated with methanol, in the presence of a suitable catalyst, to form xylenes in a reactor in a system illustrated schematically in FIG. 1, wherein a feed comprising reactants enter fluid bed reactor 11 via conduit 1 and effluent comprising product exits through conduit 5, and the catalyst circulates between fluid bed reactor 11, apparatus 12, which strips fluid from the catalyst, and catalyst regenerator 13, via conduits 2, 3, and 4, respectively. Water is typically co-fed with toluene and methanol to minimize toluene coking in the feed lines and methanol self-decomposition. Other side reactions include the formation of light olefins, light paraffins, as reactions that convert para-xylenes to other xylene isomers or heavier aromatics.
Although toluene methylation, and particularly the para-selective toluene methylation process of U.S. Pat. No. 6,504,072, provides an attractive route to para-xylene, the process inevitably produces significant quantities of C1-C5 non-aromatics. Therefore, it is desirable to suppress the formation of C1-C5 non-aromatics and produce more aromatics in the toluene methylation reaction.